The best bet is to use a belt sander or sandpaper, to get as smooth a finish as possible and wiper clean when finished, then use an undercoat, this will even out the texture of the wood. When the undercoat is dry, slowly build up layers of the spray paint, until it is all even and smooth, with no runs. Take your time with this, the quicker you rush it, the worse it will look. For an average size mantle, I would say at least 2-3 cans of paint for an top quality finish. Leave it to dry for 20-30 minutes between coats, or whatever the can says. Lastly, once you have the coverage you want, finish it off with a high gloss clear lacquer.
Hope this helps!
